There is provided herein a catalyst and a method for its preparation. There is also provided herein an isoparaffin-olefin alkylation process carried out in the presence of this catalyst to provide an alkylate product useful, inter alia, as an octane enhancer for gasoline.
As a result of the curtailment in the use of tetraethyl lead as an octane-improving additive for gasoline, not only has the production of unleaded gasoline increased but the octane number specification of all grades of gasoline has increased as well. Isoparaffin-olefin alkylation is a key route to the production of highly branched paraffin octane enhancers which are to be blended into gasolines.
Alkylation involves the addition of an alkyl group to an organic molecule. Thus, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, alkylation often involves the reaction of C.sub.2 -C.sub.5 olefins with isobutane in the presence of an acidic catalyst. Alkylates are valuable blending components for the manufacture of premium gasolines due to their high octane ratings.
In the past, alkylation processes have included the use of hydrofluoric acid or sulfuric acid as catalysts under controlled temperature conditions. Low temperatures are utilized in the sulfuric acid process to minimize the undesirable side reaction of olefin polymerization and the acid strength is generally maintained at 88-94 percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature-sensitive and the acid is easily recovered and purified.
The typical types of alkylation currently used to produce high octane gasoline blending component, that is, the hydrofluoric acid and sulfuric acid alkylation processes, have inherent drawbacks including environmental concerns, acid consumption and disposal of corrosive materials. With the increasing demands for octane and the increasing environmental concerns, it has been desirable to develop an alkylation process based on a solid catalyst system. The catalyst of the present invention offers a refiner a more environmentally acceptable alkylation process than the currently used hydrofluoric and sulfuric acid alkylation processes.
Crystalline metallosilicates, or zeolites, have been widely investigated for use in the catalysis of isoparaffin-olefin alkylation. For example, U.S. Pat. No. 3,251,902 describes the use of a fixed bed of ion-exchanged crystalline aluminosilicate having a reduced number of available acid sites for the liquid phase alkylation of C.sub.4 -C.sub.20 branched-chain paraffins with C.sub.2 -C.sub.12 olefins. The patent further discloses that the C.sub.4 -C.sub.20 branched-chain paraffin should be allowed to substantially saturate the crystalline aluminosilicate before the olefin is introduced to the alkylation reactor.
U.S. Pat. No. 3,450,644 discloses a method for regenerating a zeolite catalyst used in hydrocarbon conversion processes involving carbonium ion intermediates.
U.S. Pat. No. 3,549,557 describes the alkylation of isobutane with C.sub.2 -C.sub.3 olefins using certain crystalline aluminosilicate zeolite catalysts in a fixed, moving or fluidized bed system, the olefin being preferably injected at various points in the reactor.
U.S. Pat. No. 3,644,565 discloses the alkylation of a paraffin with an olefin in the presence of a catalyst comprising a Group VIII noble metal present on a crystalline aluminosilicate zeolite, the catalyst having been pretreated with hydrogen to promote selectivity.
U.S. Pat. No. 3,647,916 describes an isoparaffin-olefin alkylation process featuring the use of an ion-exchanged crystalline aluminosilicate, isoparaffin/olefin mole ratios below 3:1 and regeneration of the catalyst.
U.S. Pat. No. 3,655,813 discloses a process for alkylating C.sub.4 -C.sub.5 isoparaffins with C.sub.3 -C.sub.9 olefins using a crystalline aluminosilicate zeolite catalyst wherein a halide adjuvant is employed in the alkylation reactor. The isoparaffin and olefin are introduced into the alkylation reactor at specified concentrations and catalyst is continuously regenerated outside the alkylation reactor.
U.S. Pat. No. 3,893,942 describes an isoparaffin-olefin alkylation process employing, as catalyst, a Group VIII metal-containing zeolite which is periodically hydrogenated with hydrogen in the gas phase to reactivate the catalyst when it has become partially deactivated.
U.S. Pat. No. 3,236,671 discloses the use, in alkylation, of crystalline aluminosilicate zeolites having silica to alumina mole ratios above 3 and also discloses the use of various metals exchanged and/or impregnated on such zeolites.
U.S. Pat. No. 3,706,814 discloses another zeolite catalyzed isoparaffin-olefin alkylation process and further provides for the addition of C.sub.5 + paraffins such as Udex raffinate or C.sub.5 + olefins to the alkylation reactor feed and the use of specific reactant proportions, halide adjuvants, etc. U.S. Pat. No. 3,624,173 discloses the use, in isoparaffin-olefin alkylation, of zeolite catalysts containing gadolinium.
U.S. Pat. No. 3,738,977 discloses alkylation of paraffins with ethylene employing a zeolite catalyst which possesses a Group VIII metal component, the catalyst having been pretreated with hydrogen.
U.S. Pat. No. 3,865,894 describes the alkylation of C.sub.4 -C.sub.6 isoparaffin with C.sub.3 -C.sub.9 monoolefin employing a substantially anhydrous acidic zeolite, for example acidic zeolite Y (zeolite HY), and a halide adjuvant.
U.S. Pat. No. 3,917,738 describes a process for alkylating an isoparaffin with an olefin using a solid, particulate catalyst capable of absorbing the olefin. The isoparaffin and the olefin are admixed to form a reactant stream in contact with catalyst particles at the upstream end of an adsorption zone after which the reactants are passed concurrently with the catalyst so that a controlled amount of olefin is adsorbed onto the catalyst before the combination of reactants and catalyst is introduced into an alkylation zone. This controlled olefin adsorption is said to prevent polymerization of the olefin during alkylation.
U.S. Pat. No. 4,377,721 describes an isoparaffin-olefin alkylation process utilizing, as catalyst, ZSM-20, preferably HZSM-20 or rare earth cation-exchanged ZSM-20.
U.S. Pat. No. 4,384,161 describes a process for alkylating isoparaffins with olefins to provide alkylate employing as catalyst a large pore zeolite capable of absorbing 2,2,4-trimethylpentane, e.g., ZSM-4, ZSM-20, ZSM-3, ZSM-18, zeolite Beta, faujasite, mordenite, zeolite Y and the rare earth metal-containing forms thereof, and a Lewis acid such as boron trifluoride, antimony pentafluoride or aluminum trichloride. The use of a large pore zeolite in combination with a Lewis acid in accordance with this patent is reported to greatly increase the activity and selectivity of the zeolite thereby effecting alkylation with high olefin space velocity and low isoparaffin/olefin ratio.
U.S. Pat. Nos. 4,992,615; 5,012,033; and 5,073,665 describe an isoparaffin-olefin alkylation process utilizing, as a catalyst, a zeolite designated as MCM-22. Copending U.S. application Ser. No. 07/929,550, filed Aug. 13, 1992, now U.S. Pat. No. 5,258,569, describes an isoparaffin-olefin alkylation process utilizing, as a catalyst, a material designated as MCM-36. Copending U.S. application Ser. No. 07/774,482, filed Oct. 10, 1991, now U.S. Pat. No. 5,254,792, describes an isoparaffin-olefin alkylation process utilizing, as a catalyst, a material designated as MCM-49.
The catalyst provided herein is made by an unusual method. A conventional catalyst preparation involves the combination of a zeolite material with a binder in a particular extrusion method. According to this particular method, the as-synthesized form of the zeolite mixed with a suitable binding material, such as certain forms of alumina, and extruded to form extrudate. The alumina used in this extrusion process usually includes a substantial amount of chemically bound water in the form of hydroxyl groups covalently bonded to aluminum atoms.
After the extrudate is formed, it is customary to calcine the extrudate. This calcination serves two purposes. First, the calcination serves to decompose and/or oxidize residual organic directing agent included in the as-synthesized form of the zeolite. Second, this calcination serves to improve the crush strength by a condensation reaction, whereby hydroxyl groups associated with aluminum atoms in the binder react to form Al --O--Al linkages and to liberate water.
After the extrudate is calcined, it is customary to convert the calcined form of the zeolite therein to the hydrogen form by a two step technique. This two step technique involves (1) exchanging ions in the zeolite with ammonium ions, followed by (2) subjecting the extrudate to a second calcination treatment to decompose ammonium ions.
In accordance with the catalyst preparation method provided herein, it has been discovered that, for a particular class of crystalline oxide materials, the initial calcination treatment of the initially formed extrudate may be eliminated. Surprisingly, the elimination of this initial calcination results in catalysts which have superior properties in comparison with catalysts prepared using this conventional initial calcination treatment.